As for semiconductor silicon wafers (hereafter referred to as wafer), the size of a surface waviness component called a “nanotopography” has been a problem in recent years. The nanotopography is a λ=0.2 to 20 millimeters wavelength component extracted from a surface shape of the wafer, which is shorter than “sori” or “Warp” and longer than “surface roughness”, and also a very shallow waviness with a PV value equal to or less than 0.1 to 0.2 micrometer.
The nanotopography is generally measured by an “optical interferometric” measuring instrument (brand name; Nanomapper (ADE Corp.) or Dynasearch (Raytex Corporation)), and a measurement example thereof is shown in FIG. 5. FIG. 5(a) is a nanotopography map, where intensity of the nanotopography is qualitatively shown with contrasting density. Meanwhile, FIG. 5(b) shows shapes and quantitative intensities of the nanotopography on four sections (diameters) measured for every interval of 45 degrees, and the peak and the valley of the graph respectively correspond to the dark color and the light color of the nanotopography map. Incidentally, FIG. 6 is a schematic view illustrating correspondences between the nanotopography map and nanotopography sectional shapes.
It is said that this nanotopography has an influence on a yield of STI (Shallow Trench Isolation) process in device manufacturing. The nanotopography is built up in a wafer processing process (slice to polish), and strongly influenced by a grinding processing, especially double-disc grinding.
An outline of the double-disc grinding is schematically shown in FIG. 2. A raw wafer W (sliced wafer) is inserted in a hole with substantially the same diameter as the wafer, which is perforated in a glass epoxy thin plate (not shown), and as shown in FIG. 2(a), the raw wafer is held so that there may be gaps h between each of static pressure pads 11 and 21 and itself, wherein the static pressure pads are two metal thick plates on the right and left sides of the wafer with substantially the same size as the wafer diameter. The static pressure pad has lands 13 (bank portions) and pockets 14 (concave portions) on the surface as shown in FIG. 4(a). Static pressure water is supplied to the pockets 14 and thereby the wafer W is rotatably held as shown in FIG. 2 (c). A Part of the static pressure pads are cut out as shown in FIG. 4(a) and grinding wheels 12 and 22 are inserted therein to rotate the wafer W and the grinding wheels 12 and 22 as shown in FIG. 2(b), so that the wafer W is simultaneously grinded from both the right and left sides. The wafer W rotates at several tens of rpm during grinding by driving it in such a way of pressing a driving roller against an edge or hocking claws into notches, for example. Note that the pattern of the lands 13 of a conventional static pressure pad is a concentric circle with respect to a rotation center of the wafer (it corresponds with a center of the pad) as shown in FIG. 4(a).
For the wafer, which has been ground by the double-disc grinding, the nanotopography is measured with Nanomapper or the like as described above. The resulting data is processed with an operational program, and nanotopography measurement values on four diameters of the wafer surface, that is, the nanotopography measurement values on eight radii thereof are then obtained (FIG. 7 (a)). The obtained nanotopography measurement values on the eight radii are averaged by eight point at each position in a radial direction, and an “average component” shown in FIG. 7(b) is then obtained (refer to FIG. 10 for the positions in the radial direction).
As for the average component, it is known that it can be classified into a double-disc center, a central concave-convex portion, a middle ring, an outermost circumferential ring, or the like according to a distance from the wafer center as shown in FIG. 7(b).
Conventionally, warpage may occur on the wafer ground by unbalanced cutting loads on both sides of the wafer or the like during the double-disc grinding, so that in order to suppress the occurrence of the warpage, a double-disc grinding method wherein a relative position between the wafer and the grinding wheels is adjusted has been proposed (for example, refer to International Patent Application Publication 00/67950). A specific example of such a method for adjusting positions of the wafer W and the grinding wheels 12 and 22 is shown in FIG. 8. One is called “shift adjustment”, wherein the grinding wheels 12 and 22 are vertically moved in parallel to the wafer surface (FIG. 8(a)), and the other is called “tilt adjustment”, wherein relative angles between the wafer surface and each of the grinding wheels 12 and 22 are changed (FIG. 8(b)).
Ten examples of the average components with the different adjusted amounts obtained by performing such shift adjustment and tilt adjustment on the double-disc grinding machine, measuring the nanotopography of the wafer ground with the adjusted double-disc grinding machine, and processing the nanotopography measurement values are shown in FIG. 9, where they are overlappingly drawn. It is understood from FIG. 9 that the “central concave-convex portion” and the “outermost circumferential ring” can be changed in direction and size in +/− directions by the shift adjustment or the tilt adjustment and an improvement can be achieved, while the “middle ring” is hardly changed in direction and size. As described above, although the “central concave-convex portion” and the “outermost circumferential ring” of the average components in the nanotopography could be minimized only by the conventional shift adjustment or tilt adjustment, the “middle ring” thereof could not be minimized only by it.